The Archbishop of Westminster has defended the rule under which remarried divorcees in the Catholic Church are denied access to Holy Communion unless their previous marriage was formally annulled by the Church.

He hinted that at the Pope's Synod on the Family in Rome next month, when this will be discussed, there will be a push to create a better understanding of the Eucharist rather than change the rules. This would involve educating laity to understand that they could still participate effectively in the Mass without actually receiving Communion.

Cardinal Vincent Nichols also explained why unmarried couples who cohabit might not be refused Communion, even though they are technically breaking the rules, while remarried divorcees will be.

The rule is extremely contentious and many believe it to be cruelly unjust. It means that, for example, a convicted murderer or child abuser who has confessed their crime, or "sin", to a priest and received absolution is allowed to receive the sacrament of Communion. But a woman whose husband deserted her for someone else and who falls in love again and remarries, rather than choosing to cohabit, can be denied Communion unless she goes through the tortuous steps needed for an annulment, or unless her spouse happens to die. In any case, she might not wish for an annulment if she has children and believes her first marriage was valid.

It is spelled out in Canon Law, which states: "A person bound by the bond of a prior marriage, even if it was not consummated, invalidly attempts marriage. Even if the prior marriage is invalid or dissolved for any reason, it is not on that account permitted to contract another before the nullity or dissolution of the prior marriage is established legitimately and certainly."

It is also reiterated in the Catechism, which states: "Today there are numerous Catholics in many countries who have recourse to civil divorce and contract new civil unions. In fidelity to the words of Jesus Christ – 'Whoever divorces his wife and marries another, commits adultery against her; and if she divorces her husband and marries another, she commits adultery.' The Church maintains that a new union cannot be recognized as valid, if the first marriage was. If the divorced are remarried civilly, they find themselves in a situation that objectively contravenes God's law. Consequently, they cannot receive Eucharistic communion as long as this situation persists."

Cardinal Nichols explained why allowing Communion for remarried divorcees would require a "radical rethink".

Speaking at a press conference in London, he said: "This is obviously a crucial part of this debate because it brings together two very significant aspects of Catholic teaching and understanding. It brings together a teaching about the indissolubility of a valid marriage ....and it also brings together a teaching about the meaning of receiving Holy Communion, that visible sharing in the Eucharist.

"These are two aspects of Catholic faith that have deep roots and deep implications. And I don't see for myself where the area of manoeuvre opens up without quite a radical rethink of one or the other. And therefore I go to this synod really intent on listening to what people have to say. Aspects of the Eucharist for example - there are many levels in which those who participate in the Mass share in the sacrifice of Christ. They have to be affirmed and explored.

"There are dimensions of the public reception of the Eucharist which commit me to a public witness of my faith. That has to be understood more clearly. I think we've grown up with receiving Holy Communion as an act of personal devotion, which I think it is. But I think it's right to say that it's also an undertaking to be the Body of Christ in the world today."

He said he was "absolutely dedicated" to the task of listening and discernment.

"What we are entering into is very central to the pontificate of Pope Francis. He has said something like this - the journey of the people of God in their encounter and towards their encounter with Christ is the hermeneutical principle by which to understand the life and action of the Church."

Asked whether priests should also make it clearer that couples who live together without being married should also be denied Communion, the Cardinal said: "When a person presents themselves for Holy Communion, the onus, the responsibility for doing so lies with that person. So as a matter of practice, priests do not refuse to give Holy Communion. Someone who's entered a second civil marriage has already made a public statement. So in a way that does put them in a different situation, because they have made a statement on public record of where their life is. And I think that's the difference."

He said that simply being present at Mass was a "sharing" in the Communion and there was more than one level on which to participate in the Eucharist.

Cardinal Nichols said he was "looking forward" to the forthcoming Extraordinary Synod on the Family, which takes place in Rome between 5-19 October 2014.

He said: "I believe Pope Francis is calling for a return to that 'lived sense' of the mercy and compassion of God, who always accompanies us. One of the challenges is to find ways to recreate a 'culture of mercy' in the Church.

"We also need to be clear that there is a distinction between that culture of mercy and the acts that are necessary for forgiveness and conversion. Mercy is the air we are to breathe - forgiveness and conversion are the pathway we are to walk."

He was speaking as Pope Francis named a prominent woman theologian from Australia as one of the new members of his influential International Theological Commission. Tracey Rowland is one of five women on the 30-strong commission, raising the number from the previous two and meaning women now make uo 16 per cent of the membership. The others are Sister Prudence Allen RSM, Sister Alenka Arko, Dr Moira McQueen and Prof Marianne Schlosser. All members serve for five years.

Professor Rowland, Dean of the John Paul II Institute for Marriage and Family in Melbourne, is author of a widely-read book on the Pope Emeritus, titled: Ratzinger's Faith: the Theology of Pope Benedict XVI.

The commission exists to help the Congregation for the Doctrine of the Faith decide matters of doctrine.

The Vatican said: "Women now constitute 16 per cent of the Commission's members, a sign of growing female involvement in theological research."